cmd/gomobile: require NDK r19c and delete workaround for NDK r19b
cmd/gomobile: require NDK r19c and delete workaround for NDK r19b

The prebuilt toolchains introduced in NDK r19b didn't work on Windows.
NDK r19c contains a fix, so remove our workaround and check for r19c
on Windows.

Change-Id: I3bcfedbc156f10c3cab8e74dcbd7de68575669e7
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/168067
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile